1. Field of the Invention
The present invention relates to a semiconductor laser device which includes a wavelength selecting structure formed in the vicinity of an active layer in a resonator structure, the wavelength selecting structure being capable of selecting a lasing wavelength xcexe independent of the optical gain distribution of the active layer, for emitting laser light of the selected lasing wavelength xcexe. In particular, the present invention relates to a semiconductor laser device which is capable of a stable single-mode lasing in a wide temperature range, has a high mode-to-side-mode suppression ratio (SMSR) at the lasing wavelength, and is best suited especially to a light source for optical communication.
2. Description of the Related Art
A distributed feedback semiconductor laser (hereinafter, referred to as a DFB laser) has in its resonator a diffraction grating for changing the real part and/or the imaginary part of the refractive index (complex refractive index) periodically, so that only the light having a specific wavelength is fed-back for wavelength selectivity.
In a DFB laser having in the vicinity of its active layer a diffraction grating including a compound semiconductor layer that periodically differs in refractive index from the surroundings, the lasing wavelength xcexDFB of the DFB laser is determined by the relation xcexDFB=2neffxcex9, where xcex9 is the period of the diffraction grating and neff is the effective refractive index of the waveguide. Thus, the period xcex9 of the diffraction grating and the effective refractive index neff of the waveguide can be adjusted to set the lasing wavelength xcexDFB independent of the peak wavelength of the optical gain of the active layer.
For example, when the lasing wavelength of the DFB laser is set at wavelengths shorter than the peak wavelength of the optical gain distribution of the active layer, the differential gain increases to improve the DFB laser in high-speed modulation characteristic and the like,
Setting the lasing wavelength of the DFB laser at around the peak wavelength of the optical gain distribution of the active layer decreases the threshold current at room temperature.
Setting the lasing wavelength of the DFB laser at wavelengths longer than the peak wavelength of the optical gain distribution of the active layer makes the temperature characteristic suitable, which improves the operational characteristics at higher temperatures as well as the high intensity output characteristics at higher temperatures or under higher current injection.
By the way, in the conventional DFB laser, the lasing wavelength, whether falling at wavelengths shorter or longer than the peak wavelength of the optical gain distribution, is set within a close wavelength range of several tens of nanometers from the peak wavelength of the optical gain distribution of the active layer. The reasons for this are that (1) the threshold current can be held down, and (2) the single-mode operation is maintained.
Moreover, in the conventional DFB laser, the compound semiconductor layer constituting the diffraction grating has a bandgap energy considerably higher than the bandgap energy of the active layer and the energy of the lasing wavelength. More specifically, the bandgap wavelength of the compound semiconductor layer constituting the diffraction grating typically resides in wavelengths 100 nm or more shorter than the lasing wavelength, and accordingly the compound semiconductor layer is transparent to the lasing wavelength, with little light absorption or loss. The diffraction grating which shows periodical, spatial changes in refractive index is fabricated by laminating the compound semiconductor layers, followed by etching to form rows of layers which extend in parallel and periodically.
Here, the conventional DFB laser will be further described in the concrete. The conventional DFB laser can be broadly divided into a first conventional example in which xcexe is 1550 nm, xcexg falls within the range of 1200 and 1300 nm, and xcexg less than xcexmax less than xcexe holds as shown in FIG. 8(a), and a second conventional example in which xcexe is 1550 nm, kg is 1650 nm, and xcexmax less than xcexe less than xcexg holds as shown in FIG. 8(b).
In the first conventional example, xcexexe2x88x92xcexg 300 nm. Meanwhile, xcexexe2x88x92xcexg=xe2x88x92100 nm in the second conventional example,
Here, the full-lined curve in FIG. 8(b) shows the optical gain distribution of the active layer with respect to the wavelength on the abscissa. The broken-lined curve is a curve showing the amount of absorption (loss) in the diffraction grating layer with respect to the wavelength on the abscissa.
In this connection, xcexe is the lasing wavelength of the DFB laser determined by the period of the diffraction grating and the effective refractive index of the waveguide, xcexg is the bandgap wavelength of the diffraction grating layer, and xcexmax is the peak wavelength of the optical gain distribution of the active layer. The bandgap wavelength of the buried layer, or typically an InP layer, in the diffracting grating layer is xcexInP (=920 nm).
Nevertheless, in the conventional DFB laser, when the space period of the diffraction grating was adjusted to set the lasing wavelength of the DFB laser at wavelengths longer than the peak wavelength of the optical gain distribution of the active layer, Fabry-Perot lasing sometimes occurred not at the set lasing wavelength of the DFB laser but at the peak wavelength of the optical gain distribution of the active layer.
Moreover, even if the DFB laser lases at the designed lasing wavelength, there is a problem that a side mode suppression ratio (SMSR) of adequate magnitude cannot be secured between the lasing mode at the designed lasing wavelength of the DFB laser and the mode around the peak wavelength of the optical gain distribution of the active layer. For example, in the conventional DFB laser, the side mode suppression ratio (SMSR), though depending on the amount of detuning to the lasing wavelength of the DFB laser, falls within a comparatively small range of 35 and 40 dB. As a result, the conventional DFB laser has a problem in that it was impossible for the lasing wavelength of the DFB laser to be enlarged in the amount of detuning with respect to the peak wavelength of the optical gain distribution of the active layer.
To be more specific, the first conventional example with the lasing wavelength xcexe of the DFB laser greater than the bandgap wavelength xcexg of the diffraction grating layer has the advantages that the absorption loss at the lasing wavelength xcexe is small, the threshold current is accordingly low, and the optical output-injection current characteristics are favorable. However, the smaller difference in refractive index between the diffraction grating layer and the InP buried layer requires a reduction of the distance between the diffraction grating and the active layer. As a result, the coupling coefficient varies greatly depending on the thickness of the diffraction grating layer and the duty ratio, which makes it difficult to fabricate same-characteristic DFB lasers with stability.
Moreover, assuming that the absorption coefficient with respect to the lasing wavelength xcexe of the DFB laser is xcex1e and the absorption coefficient with respect to the bandgap wavelength of the active layer, or the peak wavelength xcexmax of the optical gain distribution of the active layer, is xcex1max, then xcex1e ≈xcex1max≈0. This means a smaller suppression effect both in the Fabry-Perot lasing mode in the vicinity of the peak wavelength of the optical gain distribution of the active layer and in the lasing mode of the DFB laser. Accordingly, there was a problem is that the absolute value of the detuning amount |xcexexe2x88x92xcexmax| cannot be made greater since an increase in the absolute value of the detuning amount |xcexexe2x88x92xcexmax| lowers the single-mode properties of the longitudinal mode.
In the second conventional example in which the bandgap wavelength xcexg of the diffracting grating layer exceeds the lasing wavelength xcexe of the DFB laser, the greater difference in refractive index between the diffraction grating layer and the InP buried layer makes it possible to increase the distance between the diffraction grating and the active layer. As a result, the coupling coefficient is prevented from varying with the thickness of the diffraction grating layer and the duty ratio. Thus, same-characteristic DFB lasers can be fabricated with stability, to provide an advantage of high product yields.
On the other hand, the higher absorption loss with respect to the lasing wavelength xcexe results in a diffraction grating of absorption type, producing problems of higher threshold current and unfavorable optical output-injection current characteristics. Moreover, despite of xcex1e≈xcex1max greater than 0 which provides suppression effects both in the Fabry-Perot lasing mode and in the lasing mode of the DFB laser, there was a problem that the absolute value of the detuning amount |xcexexe2x88x92xcexmax| cannot be made greater since an increase in the absolute value of the detuning amount |xcexexe2x88x92xcexmax| lowers the single-mode properties of the longitudinal mode. In the above description, the problem in the peak wavelength of the optical gain distribution of the active layer and the lasing wavelength is explained by exemplifying the DFB laser. However, this problem is not limited to the DFB laser, and is commonly associated with semiconductor laser devices each including a wavelength selecting structure in the vicinity of the active layer within the resonator structure to emit laser light having a selected wavelength xcexe, the wavelength selecting structure being such that the lasing wavelength xcexe can be selected independently of the optical gain of the active layer.
In view of the foregoing and in order to solve the foregoing problems, it is a first object of the present invention to provide a semiconductor laser device which is low in the absorption loss at the lasing wavelength of the DFB laser and high in the absorption loss at the peak wavelength of the optical gain distribution of the active layer, is accordingly low in threshold current, is favorable in optical output-injection current characteristics, and can maintain favorable single-mode properties of the longitudinal mode even if the absolute value of the detuning amount |xcexexe2x88x92xcexmax | is increased. A second object of the present invention is to provide a semiconductor laser device which varies little in the coupling coefficient with the thickness of the diffraction grating layer and the duty ratio of the DFB laser, and thus is high in product yield.
A semiconductor laser device according to the present invention (hereinafter, referred to as a first invention) is a semiconductor laser device including a wavelength selecting structure formed in a vicinity of an active layer in a resonator structure, the wavelength selecting structure being capable of selecting a lasing wavelength xcexe independent of the optical gain distribution of the active layer, for emitting laser light of the selected lasing wavelength xcexe, wherein
an absorption region made of a compound semiconductor layer having an absorption coefficient xcex1max with respect to a peak wavelength xcexmax of the optical gain distribution of the active layer which exceeds an absorption coefficient ate of the absorption region with respect to the lasing wavelength xcexe, is formed in the vicinity of the active layer.
As employed in the first invention as well as the second and third inventions to be described later, a semiconductor laser device including a wavelength selecting structure formed in a vicinity of an active layer in a resonator structure, the wavelength selecting structure being capable of selecting a lasing wavelength xcexe independent of the optical gain distribution of the active layer, for emitting laser light of the selected lasing wavelength xcexe refers to, for example, a distributed feedback (DFB) semiconductor laser device, a distributed Bragg reflector (DBR) semiconductor laser device, a fiber Bragg grating (FBG) semiconductor laser module, or the like.
Moreover, the term xe2x80x9cvicinity of the active layerxe2x80x9d means existing within the range capable of detecting the light produced in the active layer.
The absorption region wherein the absorption coefficient with respect to the peak wavelength of the optical gain distribution of the active layer exceeds the absorption coefficient with respect to the lasing wavelength of the semiconductor laser device is provided so that only the mode in the vicinity of the peak wavelength of the optical gain distribution of the active layer is selectively absorbed to suppress Fabry-Perot lasing in the vicinity of the peak wavelength of the optical gain distribution of the active layer. This can improve the wavelength selectivity for the lasing wavelength and enlarge the side mode suppression ratio (SMSR) as well, thereby enhancing the single-mode properties. Thus, the product yield improves.
In other words, since the Fabry-Perot lasing in the vicinity of the peak wavelength of the optical gain distribution of the active layer is suppressed selectively, favorable single-mode properties of the longitudinal mode can be maintained even if the amount of detuning (xcexexe2x88x92xcexmax) is increased. Moreover, since the single mode can be maintained over high operating temperatures, the high-power characteristics at high in temperature is favorable.
Another semiconductor laser device according to the present invention (hereinafter, referred to as a second invention) is a semiconductor laser device including a wavelength selecting structure formed in a vicinity of an active layer in a resonator structure, the wavelength selecting structure being capable of selecting a lasing wavelength xcexe independent of the optical gain distribution of the active layer, wherein:
an absorption region made of a compound semiconductor layer is arranged in the resonator structure; and
a bandgap wavelength xcexg of the absorption region and the lasing wavelength xcexe satisfy 0 less than xcexexe2x88x92xcexgxe2x89xa6100 nm.
The absorption region employed in the second and third inventions refers to a region made of a compound semiconductor layer, of which a bandgap wavelength xcexg and the lasing wavelength xcexe satisfy 0 less than xcexexe2x88x92xcexgxe2x89xa6100 nm. It is a broad concept including not only diffraction grating layers but also compound semiconductor layers other than the diffraction grating layers. In the description of the prior art, as described above, the bandgap wavelength of the diffraction grating layer is defined as xcexg. In the second and third inventions, however, the definition of xcexg in the prior art is extended so that xcexg is defined to cover the bandgap wavelength of the absorption region.
In the second invention, given that the semiconductor laser device is a DFB laser, the bandgap wavelength xcexg of the absorption region, or the bandgap wavelength xcexg of the diffraction grating layer, and the lasing wavelength xcexe of the DFB laser satisfy 0 less than xcexexe2x88x92xcexgxe2x89xa6100 nm. Since the lasing wavelength xcexe of the DFB laser is greater than the bandgap wavelength xcexg of the diffraction grating layer, there are advantages that the absorption loss at the lasing wavelength xcexe is small, the threshold current is accordingly low, and the optical output-injection current characteristics are favorable.
Furthermore, given that the buried layer is an InP layer, the diffraction grating layer and the InP buried layer have a greater difference in refractive index as in the second conventional example, which makes it possible to increase the distance between the diffraction grating and the active layer. As a result, the coupling coefficient is prevented from varying with the thickness of the diffraction grating layer and the duty ratio. Thus, same-characteristic DFB lasers can be fabricated with stability, to provide an advantage of high product yields.
That is, the second invention has the advantages of both the first and second conventional examples.
Another semiconductor laser device according to the present invention (hereinafter, referred to as a third invention) is a semiconductor laser device including a wavelength selecting structure formed in a vicinity of an active layer in a resonator structure, the wavelength selecting structure being capable of selecting a lasing wavelength xcexe independent of the optical gain distribution of the active layer, wherein:
an absorption region is provided in the resonator structure, the absorption region being made of a compound semiconductor layer having an absorption coefficient xcex1max with respect to a peak wavelength xcexmax of the optical gain distribution of the active layer which exceeds an absorption coefficient xcex1e of the absorption region with respect to the lasing wavelength xcexe; and
a bandgap wavelength xcexg of the absorption region and the lasing wavelength xcexe satisfy 0 less than xcexexe2x88x92xcexgxe2x89xa6100 nm.
Note that the third invention has the effects of both the first and second inventions.
In the second and third inventions, the value of xcexxe2x88x92xcexg is greater than zero and equal to or less than 100 nm, more preferably, greater than zero and equal to or less than 70 nm.
Moreover, in the second and third inventions, the peak wavelength xcexmax of the optical gain distribution of the active layer satisfies either xcexg less than xcexmax less than xcexe or xcexmax less than xcexg less than xcexe as shown in FIGS. 9(a) and (b).
Satisfying xcexg less than xcexmax less than xcexe makes temperature characteristics favorable, improving the operating characteristics at high temperature as well as the high-power characteristics at high temperature or under large current injection. For example, settings are made so that xcexexe2x88x92xcexmax=20 nm and xcexexe2x88x92xcexg=50 nm. Now, satisfying xcexmax less than xcexg less than xcexe allows greater xcex1max, which is effective at suppressing the Fabry-Perot mode sufficiently. For example, xcexexe2x88x92xcexmax=20 nm, and xcexgxe2x88x92xcexmax ranges from 10 nm to 20 nm.
In the first and third inventions, the difference between the absorption coefficient xcex1max with respect to the peak wavelength xcexmax of the optical gain distribution of the active layer and the absorption coefficient xcex1e with respect to the lasing wavelength xcexe, or xcex1maxxe2x88x92xcex1e, is preferably greater for the sake of exercising the effects of the present invention. In practice, however, the effects of the present invention can be obtained from xcex1maxxe2x88x92xcex1exe2x89xa71 cmxe2x88x921  in terms of waveguide loss. More significant effects can be obtained from xcex1maxxe2x88x92xcex1exe2x89xa75 cmxe2x88x921.
It is also preferable that xcex1e is substantially zero in the absorption region, or that the absorption region is transparent to the lasing wavelength xcexe. It follows that the provision of the absorption region does not increase the waveguide loss at the lasing wavelength, nor reduce the threshold current and luminous efficiency.
Furthermore, in the first and third invention, the provision of an absorption region having a steep absorption edge owing to quantum effects, e.g. the provision of a quantum well layer, quantum fine line or a quantum dot layer having a steep absorption edge as a selective absorption region, can realize a great difference between the absorption coefficient xcex1max with respect to the gain peak wavelength of the active layer and the absorption coefficient xcex1e with respect to the lasing wavelength. Incidentally, the term xe2x80x9cquantizedxe2x80x9d as employed herein means that the compound semiconductor layer constituting the absorption region is reduced in size to a thickness on the order of quantum mechanical wavelengths of electrons so that it can exert quantum effects.
Moreover, in the first and third inventions, the wavelength selecting structure may be constituted as a diffraction grating. A selective absorption layer which functions as the absorption region may be formed in the vicinity of the active layer separately from the diffraction grating.
Incidentally, it does not matter whether the selective absorption layer is opposed to the diffraction grating across the active layer, or arranged on the same side as the diffraction grating. However, the opposite-side arrangement has a higher degree of flexibility in design since it allows arbitrary selection of the distance from the active layer.
In the first through third inventions, it is possible to increase the differential gain at high frequencies and provide favorable high-speed modulation characteristics by making the absorption region of a quantized compound semiconductor layer and setting the peak wavelength xcexmax of the optical gain distribution of the active layer to satisfy xcexe less than xcexmax with respect to the lasing wavelength xcexe as shown in FIG. 10.
According to the first invention, the absorption region made of a compound semiconductor layer having the absorption coefficient xcex1max with respect to the peak wavelength xcexmax of the optical gain distribution of the active layer which exceeds an absorption coefficient xcex1e of the absorption region with respect to the lasing wavelength xcexe is formed in the vicinity of the active layer. Thus, a DFB laser device, for example, can suppress the lasing in the Fabry-Perot mode near the peak wavelength xcexmax of the optical gain distribution of its active layer, thereby allowing a higher mode-to-side-mode suppression ratio (SMSR) at the set lasing wavelength.
Moreover, since the amount of detuning can be made greater, it is possible to maintain a stable single-mode lasing in a wide temperature range.
According to the second invention, the bandgap wavelength xcexg of the absorption region and the lasing wavelength xcexe of the DFB laser satisfy 0 less than xcexexe2x88x92xcexgxe2x89xa6xe2x88x92100 nm. Since the lasing wavelength xcexe of the DFB laser is greater than the bandgap wavelength xcexg of the diffraction grating layer, there are advantages that the absorption loss at the lasing wavelength xcexe is small, the threshold current is accordingly low, and the optical output-injection current characteristics are favorable.
Furthermore, given that the buried layer is an InP layer, the diffraction grating layer and the InP buried layer have a greater difference in refractive index, which makes it possible to increase the distance between the diffraction grating and the active layer As a result, the coupling coefficient is prevented from varying with the thickness of the diffraction grating layer and the duty ratio. Thus, same-characteristic DFB laser can be fabricated with stability, to provide an advantage of high product yields.
The third invention offers the effects of both the first and third inventions.